More important than the math...IMHO, the message sent by this move is pretty clear. You don't buy back a future royalty unless you're 100% sure that said royalty will become payable in the future. If there is any doubt about development, you use that money to advance the project.
These are smart guys, and not the type that engages in wishful thinking. They already know the inputs to the PEA. They can do the math and have a a very good handle on the most likely outcome of the PEA. They chose to buy back the nsr while it was still just a deposit, before the PEA is done, and the project becomes a future mine, and the price of the nsr goes way up.
In other words, our management is 100% positive that this will be a mine some day.